The alkylation of aromatic hydrocarbons such as benzene is a well-developed art, and one that is practiced commercially using solid catalysts in large scale industrial units. Two common commercial applications are the production of ethyl benzene and cumene (isopropyl benzene). The production of ethyl benzene is the process of alkylating benzene with ethylene to produce ethyl benzene, which is the precursor used in the production of styrene. The production of cumene is the process of alkylating benzene with propylene to form isopropylbenzene, and which is used in the production of phenol. The production of ethyl benzene and cumene have undergone continual improvement, and an example of the process and typical flow scheme is shown in U.S. Pat. No. 4,051,191.
The performance of alkylation processes is controlled by the activity and selectivity of the catalyst in the process operating environment. Catalysts used in the alkylation process are solids that have considerable acidity, such as aluminum chloride and zeolites. However, there is considerable expense associated with the loading of large quantities of catalyst into a commercial reactor. Due to the nature of aromatic alkylation catalyst deactivation, the initial catalyst loadings in commercial fixed-bed operations generally contain far more catalyst than is needed at any given time to catalyze the alkylation reaction. Only a small portion of the active catalyst sites in fresh catalyst loadings is utilized for the desired alkylation reaction, while catalyst beyond the easily recognizable alkylation exotherm is essentially not used for alkylation initially. Moreover, the active acid sites in this downstream portion of the catalyst bed tend to promote undesirable side reactions, such as the formation of oligomers and diphenyl alkanes.
The use of larger alkyl groups in the production of alkylbenzene is important for the manufacture of detergents. About thirty years ago it became apparent that household laundry detergents made of heavily branched alkylbenzene sulfonates were gradually polluting rivers and lakes. Solution of the problem led to the manufacture of detergents made of linear alkylbenzene sulfonates (LABS). The alkylbenzenes with alkyl groups that are linear biodegrade more quickly than alkylbenzenes with branched alkyl groups. It became very desirable to produce relatively pure linear alkyl benzenes for the production of detergents.
However, the chemistry is complex, and the alkyl groups are often a complex mix of isomers, as well as undergoing isomerization during the processes used in the production of linear alkyl benzenes. Control of the alkylation process to produce a product meeting purity specifications is important for the quality of the final product, or LABS.